Fix #17879: Corrected regression in python (inherited by yaml and json) serializer that prevented serializing model instances with null FK ref to a model when serializing with natural keys. Thanks danfairs and tmitchell.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17685 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/core/serializers/python.py

@@ -47,7 +47,10 @@ class Serializer(base.Serializer):
     def handle_fk_field(self, obj, field):
         if self.use_natural_keys and hasattr(field.rel.to, 'natural_key'):
             related = getattr(obj, field.name)
-            value = related.natural_key()
+            if related:
+                value = related.natural_key()
+            else:
+                value = None
         else:
             value = getattr(obj, field.get_attname())
         self._current[field.name] = value